1. Field of the Invention
The present invention relates to an image-forming apparatus such as a printer, a facsimile machine, an electrophotographic apparatus, and a copying machine in which images are formed by controlling the bias voltages for a toner-supplying roller, a developing roller, and a charging roller.
2. Description of Related Art
FIG. 13 illustrates a general configuration of a conventional image-forming apparatus.
A charging roller 4 charges the surface of a photoconductive drum 1 to a predetermined potential. An LED head 26 illuminates the charged surface of the photoconductive drum 1 to form an electrostatic latent image on the photoconductive drum 1. A toner-supplying roller 3 delivers an appropriate amount of the toner 9, supplied from the toner cartridge 12, to developing roller 2. A toner blade 10 forms a toner layer having a uniform thickness on a developer roller 2. The developing roller 2 causes toner 9 as a developer to adhere to the electrostatic latent image formed on the photoconductive drum 1, thereby forming a toner image. A transfer roller 5 transfers the toner image formed on the photoconductive drum 1 onto a print medium 11. A cleaning roller 7 removes residual toner on the surface of the photoconductive drum 1 after transferring. For ease of maintenance, the developing roller 2 and toner-cartridge 12 are usually provided in an EP cartridge 13.
The developing roller 2, toner-supplying roller 3, and charging roller 4 receive negative voltages Vg, Vs and Ve, respectively. In this specification, these negative voltages will be described by omitting their polarity. That is, “a high voltage” means “a negative voltage having a large absolute voltage value.” Likewise, “a low voltage” means “a negative voltage having a small absolute voltage value.”
With the aforementioned conventional image-forming apparatus, the charging characteristic of the toner, toner-supplying roller 3, and developing roller 2 varies with environmental conditions such as temperature and humidity in a toner cartridge 12. For the same bias voltage, the amount of toner deposited to a unit area of the developing roller 2 varies greatly. Sometimes, a total amount of charge per unit area (referred to as toner potential hereinafter) in relation to the surface potential of the photoconductive drum 1 falls outside of an appropriate range.
For example, when the charging characteristic of toner is improved due to environmental changes, more toner is deposited on the developing roller 2 and the toner potential near the developing roller 2 increases as well. Toner potential is high in non-image areas on the photoconductive drum 1 where negative charges are not dissipated by exposure. Too high a toner potential may cause the toner to adhere to the non-image areas on the photoconductive drum, resulting in soiling of the print medium 11. Conversely, when the charging characteristic degrades, less toner is deposited to the developing roller 2, so that the toner potential near the developing roller 2 decreases. Thus, the toner density of an image becomes low to cause blurred print results.
In order to address variations of toner potential due to environmental conditions, the conventional image-forming apparatus has a table that lists bias voltages for the charging roller 4 and environmental conditions corresponding to the bias voltages. For various environmental conditions, suitable bias voltages for the charging roller 4 are determined experimentally. When a printing operation is performed, a bias voltage is read from the table according to environmental conditions detected with, for example, a temperature sensor and a humidity sensor.
However, with the aforementioned conventional image-forming apparatus, if the sensors are not disposed at proper locations within the EP cartridge 13, detected environmental conditions have errors, making it difficult to set appropriate bias voltages. Additionally, the charging characteristic of toner varies with time and from cartridge to cartridge. However, the same bias voltage is read from the table for the same environmental condition. Therefore, it is difficult to set optimum bias voltages.